1) Field of the Invention
The present invention relates to a gas turbine combined plant, and more particularly to a gas turbine combined plant capable of improving combined efficiency as compared with that of a conventional plant, and a method of operating the same.
2) Description of the Related Art
A gas turbine combined plant for making effective use of thermal energy of exhaust gas has been recently constructed. The thermal energy is obtained by recovering thermal energy contained in high-temperature exhaust gas of a gas turbine by a heat recovery steam generator (HRSG) to drive a steam turbine. FIG. 12 is an explanatory diagram showing a conventional gas turbine combined plant. In this gas turbine combined plant 800, thermal energy contained in exhaust gas of a gas turbine 820 is recovered by an HRSG 810 to generate steam, the steam is supplied to a steam turbine 860 connected with a generator 868 to thereby drive the generator 868, and thereby electric power is generated.
Aiming at further improvement in combined efficiency, a regenerative type gas turbine combined plant has been used. In this gas turbine combined plant, a regenerator uses exhaust gas of a gas turbine for heating of combustion air. FIG. 13 is an explanatory diagram showing a conventional regenerative type gas turbine combined plant. This regenerative type gas turbine combined plant has a regenerator 930, to which exhaust gas of a gas turbine 920 is supplied. A temperature of combustion air is then increased by heat exchange between the exhaust gas of the gas turbine 920 and combustion air compressed by a compressor 922. As a result, a temperature of the combustion air at an inlet of a combustor 923 can be increased, and hence an amount of fuel supplied to the gas turbine 920 can be reduced. Therefore, efficiency of the gas turbine and efficiency of the combined plant can be increased. With such structure, combined efficiency in the gas turbine combined plant has been improved to the latter half of 50%-60% range, on a lower heat value (LHV) base.
In regenerative type gas turbine combined plant 900, a temperature of steam generated by HRSG 910, that is, a temperature at an inlet of the steam turbine decreases, thereby improvement in combined efficiency by regeneration is suppressed. FIG. 14 is a Q-T diagram showing a relationship between a steam temperature and a quantity of heat exchange in a conventional HRSG. The line showing higher temperature expresses changes of exhaust gas, and the line showing lower temperature expresses changes of steam (water). An area indicated by diagonal lines expresses a magnitude of loss of heat exchange, and as this area increases, the loss of heat exchange increases.
As seen from FIG. 14, in this regenerative type gas turbine combined plant 900, a steam temperature at an inlet of a high-pressure superheater 940 cannot be sufficiently high, and therefore loss of beat exchange of the high pressure superheater 940 increases. Further, since a temperature difference between an inlet temperature and an outlet temperature of exhaust gas in a high-pressure evaporator 950 is large, loss of heat exchange in the high-pressure evaporator 950 cannot be sufficiently reduced.
It is an object of the present invention to provide a gas turbine combined plant capable of improving at least one of gas turbine efficiency and combined efficiency as compared with a conventional gas turbine combined plant, and also a method of operating the plant.
The gas turbine combined plant according to one aspect of this invention comprises a gas turbine, an exhaust heat recovery unit that recovers thermal energy contained in an exhaust gas of the gas turbine in a heat recovery stage having at least a superheating unit and an evaporation unit, and an air heating unit that heats combustion air of the gas turbine by the exhaust gas. The plant also comprises an exhaust gas branch line that has a branch portion for branching the exhaust gas and supplies the exhaust gas to the exhaust heat recovery unit and the air heating unit, and an exhaust gas supply line that supplies the exhaust gas, after heating the combustion air of the gas turbine in the air heating unit, to between the superheating unit and the evaporation unit.
The gas turbine combined plant according to another aspect of this invention comprises a gas turbine, an exhaust heat recovery unit that recovers thermal energy contained in exhaust gas of the gas turbine by providing a plurality of heat recovery stages respectively having at least a superheating unit and an evaporation unit, and air heating units in a plurality of stages that heat combustion air of the gas turbine by the exhaust gas. The plant also comprises an exhaust gas branch line that has a branch portion for branching the exhaust gas and supplies the exhaust gas to the exhaust heat recovery unit and the air heating units in the plurality of stages, a first-stage exhaust gas supply line that supplies the exhaust gas, after heating combustion air of the gas turbine in a first-stage air heating unit, to between the superheating unit and the evaporation unit provided in a first heat recovery stage, and a subsequent-stage exhaust gas supply line that supplies the exhaust gas, after heating the combustion air of the gas turbine in the air heating unit(s) in and after a second stage, to between the superheating unit and the evaporation unit provided in the heat recovery stage(s) in and after a second heat recovery stage.
A method of operating a gas turbine combined plant according to still another aspect of this invention, comprises reducing a flow rate of exhaust gas supplied to an air heating unit as a temperature of the exhaust gas increases, at a time of operating the gas turbine combined plant including a gas turbine, an exhaust heat recovery unit that recovers thermal energy from exhaust gas of the gas turbine, and the air heating unit that heats combustion air of the gas turbine by a part of the exhaust gas of the gas turbine.
The method of operating a gas turbine combined plant according to still another aspect of this invention, comprises reducing a flow rate of exhaust gas supplied to an air heating unit as a load of the gas turbine combined plant increases, at a time of operating the gas turbine combined plant including a gas turbine, an exhaust heat recovery unit that recovers thermal energy from exhaust gas of the gas turbine, and the air heating unit that heats combustion air of the gas turbine by a part of the exhaust gas of the gas turbine.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.